The specific objective of this study is to understand by what mechanism potentially harmful maternal IgG antibodies with specificity for fetal antigens are selectively depleted from the total IgG population during passage through the placenta. It should be possible to determine whether or not IgG anti-fetal antibody is selectively removed and deactivated by cells of the chorionic villi. Studies will be performed to determine whether placental IgG is attached to Fc receptor positive cells in the chorionic villi, whether that IgG represents antigen-antibody complexes, and further if it represents antigen-antibody complexes which are formed between soluble fetal antigens and potentially harmful maternal anti-fetal antibodies. The significance rests in the potential demonstration that the placental tissue functions in a manner similar to organs of the reticuloendothelial system in its ability to filter antigen-antibody complexes from the blood.